1. Field of the Invention
This invention relates to a method of determining the chemical composition of apatite grains contained within rock samples taken from the bore of a well being drilled or from the surface of the Earth.
2. Description of the Prior Art
Numerous methods of analyzing the chemical composition of minerals have been used including the currently used method of using a microprobe to focus a beam of electrons onto the surface of a material and causing X-rays to be generated. By carefully monitoring the X-rays emitted during the bombardment of a surface by the electron beam, it is possible to determine the chemical composition of the material being bombarded.
Presently, in the oil exploration field, it is desirous to identify apatite grains contained within rocks which are rich in fluorine in order to perform analyses on them for purposes of determining the thermal history of the rock. This is most often accomplished by grinding or crushing the rock sample to obtain sand-sized grains and then separating the grains containing apatite crystals from the surrounding grains of minerals and rock in a multiple stage process using the density and magnetic characteristics of the surrounding minerals.
After a multiple step process, the remaining grains of the rock sample consist of a sufficiently high percentage of apatite for purposes of analysis. A representative portion of the remaining apatite grains is then incorporated into an epoxy wafer attached to a petrographic slide, polished to expose internal surfaces, and etched with acid. The epoxy wafer is covered with a muscovite mica detector in the form of a mask and placed adjacent to the core of a nuclear reactor along with a uranium-doped glass covered with a second muscovite mica detector where both are irradiated with thermal neutrons. The epoxy wafer, uranium-doped glass, and their attached muscovite mica detectors are then removed from the reactor and the muscovite mica detectors are immersed in hydrofluoric acid to etch the induced fission tracks caused by the induced fission of uranium in the apatite grains and the uranium-doped glass. The concentration of .sup.238 U and the fission track density per unit volume are determined for a volume of an apatite grain beneath a selected area of the apatite grain and the fission track age for each grain is determined.
In the methods currently practiced within the industry, the chemical composition of the apatite grains is then determined by a process known as microprobe analysis. This process consists of placing the apatite grains under an electron beam thereby inducing each affected apatite grain to produce X-rays. Through the careful monitoring and detection of these emitted X-rays, it is possible to determine whether the apatite grain being subjected to the electron beam is fluorine-rich, chlorine-rich, or water-rich, or some combination of these types.
The currently practiced process of determining the chemical composition of the apatite grains is an extremely costly, time consuming, and labor intensive process, and, as such, it is rarely done with sufficient completeness. Additionally, the level of expertise required of the person performing the actual steps of the analyses currently used is greater than in that of the present invention. The method of the present invention demands a high level of expertise only during the interpretation phase rather than during the actual performance of the analyses, consumes less time for a given number of samples, and results in greatly increased capital savings.
Read, U.S. Pat. No. 1,799,604 discloses a method and apparatus for identifying precious gems and crystals which operates upon the principle of an initial ray or beam of light striking a diamond and being reflected or refracted into secondary rays of light whose intensity and direction are dependent upon the angles, faces and imperfections in the diamond. This apparatus allows the recording of the secondary rays so that the diamond or crystal can be identified thereby under identical conditions.
Grayson, U.S. Pat. No. 4,093,420 discloses a method of prospecting for accumulations of minerals based upon organic material present in the rock samples taken at differing locations and depths. This method is based upon the amount of light emitted or absorbed by the specific organic particles within the rocks and the gradients between samples taken at the same location but at different depths are plotted on a map. By repeating this procedure for numerous locations, the contours which will appear on the map will encircle the mineral deposit.
Trossarelli, U.S. Pat. No. 4,906,093 discloses an illuminator device for the spectroscopic observation of samples wherein the substance under examination is illuminated by a source of white light and possesses optical fibers for transmitting the residual illumination light passed through the substance observed to an observation spectroscope.
Dobrilla, U.S. Pat. No. 4,925,298 discloses a method for measuring and plotting the etch pit density on the surface of an etched test wafer. In this method, a beam of light is focused onto an etched wafer and the intensity of the light reflected is compared with the intensity of a reference wafer to calculate the etch pit density of the etched wafer. This procedure is repeated for different areas of the etched wafer so that any variances within each wafer may be detected.
In the method of the present invention, the chemical composition of the apatite grains is determined by taking measurements of etch figures formed by the intersection of etched naturally occurring fission tracks or other crystallographic imperfections, such as other charged-particle tracks, defects, dislocations, fluid inclusions, mineral inclusions, polishing scratches, and fractures, with the planar surface of the apatite grain being observed. The purpose of the measurements is to determine if the apatite is of a fluorine-rich, chlorine-rich, or water-rich nature. Apatite grains which are fluorine-rich are identified by the characteristic dimensions of the etch figures within their etched planar surfaces.
The dimensions of the etch figures in fluorine-rich apatite and relatively non-fluorine-rich apatite are taken and, together with other information gathered by methods of analysis already used within the geological sector of the scientific community, the pooled fission track ages and the pooled distributions of perceived track lengths pertaining to the fluorine-rich apatite grains and the relatively non-fluorine-rich apatite grains, respectively, are determined.
While the prior art discloses methods and apparatus with which to observe mineral or rock samples and even calculate the density of etch pits on the surface of a wafer containing crystals which has been etched, the actual use of the dimensions of the resulting etch pits and the etch figures they form has not been practiced to determine chemical composition of the crystalline structure which has been etched.